1. Field of the Invention
The present invention relates to an aperture device suitably used in a photographing system, e.g., a video camera, a still video camera, or the like, a ND filter used therein, and a method of manufacturing the ND filter.
2. Related Background Art
An aperture device is arranged to control the amount of light incident on a silver chloride film or a solid-state imaging element such as a CCD, and when an object field is bright, the aperture device is stopped down to have a smaller aperture size. On a very fine day, or when a high-luminance object is to be photographed, the aperture device is stopped down to have a small aperture size, and is easily influenced by the hunting phenomenon of the aperture and diffraction of light, resulting in deterioration of the image. As a countermeasure against the deterioration, a film-like ND (neutral density) filter is attached to each aperture blade so as to increase the aperture size of the aperture device to some extent even when an object field is bright.
In recent years, the darkness of the ND filter is increased according to an increase in sensitivity of an imaging element so as to further decrease a transmission rate of light, so that a certain aperture size of the aperture device can be assured even when an object field is bright. However, when the darkness of the ND filter is increased, a difference between the amount of light t, which is transmitted through a ND filter 11, and the amount of light b which is not transmitted through the ND filter, is increased, and the resolving power is undesirably decreased, in a state shown in FIG. 7. In order to eliminate this drawback, the ND filter must have a structure wherein its darkness is changed so as to sequentially increase the transmission rate toward the center of the optical axis. Note that the present applicant proposed the ND filter having a structure wherein its darkness is changed so as to sequentially increase the transmission rate toward the center of the optical axis in Japanese Patent Application No. 4-77349.
In FIG. 7, a photographing optical system 6 is constituted by lenses 6A, 6B, 6C, and 6D, and a solid-state imaging element 7 is arranged in front of a low-pass filter 8. An aperture device is constituted by a ND filter 11, aperture blades 12 and 13, which are moved to oppose each other, and an aperture blade support plate 14. The two aperture blades form a substantially rhombic aperture. The ND filter is normally adhered to one of the two aperture blades.
In the conventional aperture device, as shown in FIG. 8, the ND filter 11 is adhered to the edge portion of an aperture portion 12a of the aperture blade 12 by an adhesive 15 to cover an aperture size as small as F8 to F11 so as to attain small-aperture correction. The conventional ND filter 11 adopts a uniform darkness filter.
In general, as a method of manufacturing a ND filter, a method of mixing and kneading a light absorbing pigment in a film-like material (cellulose acetate. PET, or the like), and a method of coating a light absorbing pigment onto the material are available. With these manufacturing methods, a filter having a uniform darkness can be manufactured. However, it is very difficult to manufacture a filter whose darkness changes.